I know about the “June Drop”, but my apples (Worcester Pearmain), growing against a South facing wall, keep dropping until they are all gone, long before they are ripe, or big enough. Is this a deficiency?
Against a wall, the possibility is the roots too dry, often a cause of fruit drop. Apple trees on a wall are on a weak rootstock and may need regualr watering.
Try a mulch now, some feeding 70g/square metre with fruit fertiliser and watering in late summer.
If this does not work, virus disease may be the problem, especially if the fruit is small. There is no remedy for virus.